The disclosure relates to alkali aluminosilicate glasses that have low softening points. More particularly, the disclosure relates to such glasses that are ion exchangeable and formable into three-dimensional shapes. Even more particularly, the disclosure relates to alkali aluminosilicate glasses having low softening points and containing low amounts or are free of B2O3.
Ion exchangeable glasses are widely used as cover glass for displays found in many modern electronic devices including hand-held devices. However, the use of these chemically-strengthenable glasses in such applications has been limited to devices that are, for the most part, flat and planar. The formation of three-dimensional (3D) glass shapes is in some instances accomplished by molding or vacuum sagging processes in which the glass is heated and allowed to sag under vacuum into a mold to obtain its final or near-final shape.
Many ion-exchangeable glasses, however, have softening points that are sufficiently high that such glasses tend to react with, stick to, and/or degrade the molds used for the sagging process, even when a protective coating is employed. Boron oxide (B2O3) is sometimes added to the glass to reduce the viscosity and liquidus temperature of the glass. However, B2O3 also acts to inhibit the ion exchange performance of the glass, especially when B2O3 is present in its tetrahedrally coordinated state, which can occur when alkali metal oxides are present in relatively large amounts.